Switchable color filters have been known heretofore to be positioned in front of black and white cathode ray tubes of field sequential color display systems to produce light images in full color. One type of display system utilizes birefringent materials in an attempt to provide a display with an acceptable color contrast ratio.
One such display system is disclosed in U.S. Pat. No. 2,638,816 of Stolzer who describes an adapter which receives light emitted from a black and white television set and produces a light image in different colors. The adapter includes passive birefringent sheets which develop light of different colors. A half-wave optical retardation device operates as a light gate which transmits light of the desired color in the proper sequence in synchronism with the frames of image information appearing on the black and white television screen.
One disadvantage inherent in the device of Stolzer is that the colors developed from the transmission of light through passive birefringent sheets are generally impure and vary in appearance as a function of the viewing angle. The adapter also employs interdigitated electrodes which present a pattern of lines across the display screen. The device of Stolzer, therefore, produces color images which are not acceptable for most image display applications.
U.S. Pat. No. 4,003,081 of Hilsum et al. describes a color display system which incorporates an optically active twisted nematic liquid crystal cell that includes a separate layer of passive birefringent material to produce light of different colors. Light of the desired color is selectably transmitted through a system of neutral polarizers in response to a change in the frequency or intensity of an electric field applied to the liquid crystal cell.
U.S. Pat. No. 4,019,808 of Scheffer describes a color display system which is similar to that of Hilsum et al. Scheffer employs a twisted nematic liquid crystal cell in combination with a passive birefringent optical retardation plate or sheet to produce light of different colors. Both of the display systems described by Hilsum et al. and Scheffer produce light images which exhibit colors of an impure quality that is generally characteristic of the colors developed from passive birefringent materials. The twisted nematic liquid crystal cells are utilized in both systems to rotate the plane of polarization of light passing therethrough.
U.S. Pat. No. 4,097,128 of Matsumoto et al. describes a color display device which incorporates a tunable birefringent liquid crystal cell that functions as a variable optical retarder whose birefringence is changed to develop light of various colors. The liquid crystal cell is included in an assembly of neutral polarizers which transmit light of different colors in response to a change in the birefringence of the liquid crystal cell to provide an image in full color. The change in birefringence is accomplished by varying the magnitude of a voltage which is applied to the cell. As shown in FIG. 4 of the patent, the maximum transmittance of light of a particular primary color is not accompanied by a minimum transmittance of light of the remaining primary colors. The Matsumoto et al. device is incapable, therefore, of providing light of pure quality. Moreover, devices using the birefringence properties of a liquid crystal material to produce light of different colors typically exhibit slow switching responses and limited viewing angles.
Another type of color display system employs liquid crystal materials which include pleochroic dyes to produce light of different colors. For example, U.S. Pat. No. 3,703,329 of Castellano describes such a color display system which comprises three cells each including a solution consisting of a pleochroic dye in a nematic liquid crystal composition. The alignment of the liquid crystal molecules causes alignment of the pleochroic dye dissolved therein. An electric field applied to the cell alters the alignment of the liquid crystal molecules and thereby alters the alignment of the dye in the liguid crystal combination to change the color of light observed at the output of the device.
Since the pleochroic dye is absorbed along one axis of the liquid crystal cell, an output state of white light can be provided when the electric field causes the molecules of the three liquid crystal cells to align maximally in a planar relationship.
The colored light developed from liquid crystal cells having pleochroic dyes suffers from poor color contrast. Expensive measures must be undertaken to increase the optical density of the cell to provide the desired color saturation.
Anotner type of color display system utilizes optical assemblies which incorporate liquid crystal polarization switching devices in combination with color selective linear polarizing filters. An example of such a display system is disclosed in IBM Technical Disclosure Bulletin, Vol. 22, No. 5, Oct. 1979, pp. 1769-1772, "Liquid Crystal Apparatus for Converting Black and White CRT Display into Colored Display" by A. N. Brinson and A. D. Edgar who describe a switchable color filter that incorporates liquid crystal devices for converting a black and white cathode ray tube image into a full color image.
A light output of selectable colors is achieved by Brinson et al. through the use of three linear polarizing filters. Each one of two twisted nematic liquid crystal cells is disposed between a different pair of adjacent ones of the three polarizing filters to form an assembly which is optically coupled to a black and white cathode ray tube display. One of the polarizing filters comprises a neutral linear polarizer which is positioned adjacent the cathode ray tube display screen to pass linearly polarized white light through only one polarization axis. Each of the other polarizers constitutes a color selective polarizing filter having orthogonally aligned polarization axes which pass light of different colors. Voltages are applied to the liquid crystal devices in a predetermined sequence to produce a display in which three colors are cyclicly displayed to form an image in full color.
The system described hereinabove suffers from the disadvantage of requiring the use of a neutral linear polarizer at the input end of the switchable color filter and is incapable of producing an output state of white light.
That a color display system could be provided with the use of a switchable color filter which comprises three twisted nematic liquid crystal pularization rotating switches in combination with four linear polarizing filters including three color selective and one neutral linear polarizing filters was known to the inventor prior to his invention of the display system disclosed herein. This prior art color display system is shown in FIG. 1.
With reference to FIG. 1, the prior art color display system includes a switchable color filter which comprises three spaced-apart twisted nematic liquid crystal polarization rotating switches or devices 10, 12, and 14 which are optically coupled to and disposed between three pairs of linear polarizing filters 16 and 18, 18 and 20, and 20 and 22, respectively. Each one of the linear polarizing filters has orthogonally aligned transmission and absorption axes. Polarizing filter 16 is a neutral polarizer which transmits linearly polarized white light through its transmission axis 24 and no light through its absorption axis 26. Each one of polarizing filters 18, 20, and 22 is a color selective linear polarizer which passes white light through its transmission axis and passes through its absorption axis light of a color that is different from that passed by the other color selective polarizers.
In an exemplary embodiment, color selective polarizers 18, 20, and 22 pass white light through transmission axes 28, 30, and 32, respectively, and light of the primary colors red, green, and blue through absorption axes 34, 36, and 38, respectively. The presence of a voltage signal of the proper magnitude across any one of liquid crystal devices 10, 12, and 14 leaves unchanged the plane of polarization of the light rays passing through the device. This is referred to as the "ON" state of the device. The absence of a voltage signal across any one of the liquid crystal devices imparts a 90.degree. rotation to the plane of polarization of light passing through the device. This is referred to as the "OFF" state of the device.
In operation, the switchable color filter formed by liquid crystal devices 10, 12, and 14 and polarizers 16, 18, 20, and 22 is positioned in front of light image generator 40 which emits white light from phosphor screen 42. Image generator 40 is typically a black and white television set. Neutral polarizing filter 16 receives the light emitted from screen 42 and transmits the light through its transmission axis 24 to the remaining components of the switchable color filter. Light rays which exit the switchable color filter through polarizing filter 22 in the colors red, green, or blue are optically processed in accordance with the switching states of liquid crystal devices 10, 12, and 14 as indicated in Table I.
TABLE I ______________________________________ Net Color Net Color Through Through State of State of State of Transmission Absorption Liquid Liquid Liquid Axis 32 of Axis 38 of Crystal Crystal Crystal Polarizing Polarizing Device 10 Device 12 Device 14 Filter 22 Filter 22 ______________________________________ ON OFF ON RED -- OFF OFF OFF GREEN -- OFF ON OFF -- BLUE OFF ON ON WHITE -- ______________________________________ "OFF" state imparts 90.degree. rotation to plane of linearly polarized light. "ON" state imparts no rotation to plane of linearly polarized light.
A light image in full color is produced at the output of polarizing filter 22 when the switchable color filter is switched in synchronism with the frames of image information appearing on the black and white television screen 42. As indicated in Table I, an output state of white light is obtainable with this display system.
The above-described switchable color filter is expensive in that it incorporates seven optical components and necessitates the use of complex driver circuitry to actuate each of the three liquid crystal switches in the proper sequence to provide a light output of the desired color. Moreover, a switchable color filter which includes such a relatively large number of optical components attenuates the intensity of light passing therethrough and thereby produces an image of diminished brightness.